Field of the Invention
The invention relates to an apparatus for driving at least one capacitive actuator, in particular for a fuel injection valve of an internal combustion engine. It also relates to a method for driving the apparatus.
In the known apparatuses for driving capacitive actuators, the charging process includes a ring around process of charging from a charge source via a charging coil to the actuator, with the inductance of the charging coil, together with the capacitances of the charge source and of the actuator, governing a time constant of a charging process and a discharge process (the charging time and discharging time).
An apparatus for driving a capacitive actuator is known from U.S. Pat. No. 5,130,598. In the patent a piezoelectric actuator is charged and discharged from a power source via a charging switch and a coil, using voltage pulses that are dependent on the current flowing through the actuator and the voltage applied to it. The power source must be able to supply at least one voltage corresponding to the maximum actuator voltage that, together with the complex circuit, makes this highly costly.
Patent Abstracts of Japan vol. 017, No. 126(M-1381), Mar. 17, 1993 and JP04308338 A (Nippondenso Co Ltd.), Oct. 30, 1992, discloses an apparatus for charging a capacitive actuator by a transformer. The charging current rises in an unregulated manner and, after reaching a predetermined value, is kept constant by pulse width modulation and the discharge being effected in an unregulated manner, since a resistor via which an acknowledge signal could be effected is not configured as a current measuring shunt.
Patent Abstracts of Japan vol. 012, No. 454(M-769), Nov. 29, 1988 and JP63183250 A (Toyota Motor Corp) Jul. 28, 1988, describes a control circuit for a piezoelectric actuator, in which a pulsed operation is not mentioned. A current measurement that could enable a current regulation is neither present on the primary side nor the secondary side.
U.S. Pat. No. 5,543,679 describes a drive circuit for piezoelectric actuators used in fuel injection valves according to the resonance method, the actuator being charged in a single ring around process of charging via a transformer. The discharge is effected either by dissipating the energy via a further connected coil without energy recovery (FIGS. 1, 7 and 8) or with energy recovery with additional components (FIG. 10: transformer, capacitor, diodes and additional coil).
U.S. Pat. No. 5,479,062 describes a drive circuit for piezoelectric actuators of matrix printers. The driving is effected by a transformer according to the resonance method. Since the needles of such a matrix printer are always actuated as quickly as possible and with the same force, yet not only serially but also in parallel, i.e. simultaneously, there are present for all the piezoelectric actuators a single voltage detector and only one regulating circuit, which, at intervals, monitor the piezoelectric voltage of each individual actuator and adjust the charging current in a manner dependent thereon. However, owing to the parallel driving, each piezoelectric actuator requires a dedicated drive circuit with a transformer and also a charging and discharge switch. The waveform of the charging voltage is not variable; all that is affected is a ring around process with a charging current that brings about a specific actuator voltage. No energy recovery takes place either, since the energy is dissipated in the secondary circuit.
It is accordingly an object of the invention to provide a method and a circuit for driving at least one capacitive actuator which overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, in which the charging and discharging of at least one capacitive actuator can be performed with largely freely selectable quantities for duration and profile of the charging/discharge process.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for driving at least one capacitive actuator. The method includes charging the capacitive actuator with a pulse-width-modulated charging current having a predeterminable profile of frequency or a duty ratio with a pulse sequence being continuous or predetermined by omission of individual pulses. The capacitive actuator is discharged with a pulse-width-modulated discharge current with a further predeterminable profile of frequency or a further duty ratio with the pulse sequence being continuous or predetermined by omission of individual pulses.
In accordance with an added mode of the invention, there is the step of determining the duty ratio by way of a pulse duration, which lasts until the pulse-width-modulated charging current reaches a predeterminable value.
In accordance with an additional mode of the invention, there is the step of determining the further duty ratio by way of a pulse duration, which lasts until the pulse-width-modulated discharge current reaches a predeterminable value.
In accordance with a further mode of the invention, there is the step of charging the capacitive actuator from a power source being a DC/DC converter with an adjustable output voltage, and a charging capacitor is connected in parallel with the power source.
In accordance with another mode of the invention, there is the step of charging the capacitive actuator until an actuator voltage associated with a specific charge, a specific amount of energy or a specific change is reached.
In accordance with another added mode of the invention, there is the step of using the capacitive actuator for actuating at least one fuel injection valve of an internal combustion engine.
In accordance with another additional mode of the invention, there is the step of setting the duty ratio and the further duty ratio to be substantially equivalent.
With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for controlling at least one actuator. The apparatus includes a control circuit, a power source having a first pole and a second pole, a charging switch connected to the control circuit, a discharging switch connected to the control circuit, a first diode, and a second diode. A transformer is provided and has a primary coil with a first end connected to the first pole of the power source and a second end connected to both the charging switch and the first diode. The first diode is forward-biased toward the primary coil. The transformer has a secondary coil with a first end connected to the actuator and a second end connected to both the discharge switch and to the second diode. The second diode is forward-biased toward the secondary coil. A first measurement device for measuring a primary-side charging current is disposed between the charging switch and the second pole of the power source. The first diode is connected in parallel with a first series circuit containing the charging switch connected in series with the first measurement device. A second measurement device for measuring a secondary-side discharge current is disposed between the discharge switch and the second pole of the power source. The second diode is connected in parallel with a second series circuit containing the discharge switch connected in series with the second measurement device.
Instead of being connected in series circuits, the first diode can be connected in parallel with the charging switch and the second diode can be connected in parallel with the discharge switch.
In accordance with an added feature of the invention, a third measurement device for measuring a secondary-side charging current is disposed in a third series circuit with the actuator.
In accordance with another feature of the invention, a selection switch is connected in series with the actuator, and a third measurement device for measuring a secondary-side charging current is disposed in a series with the selection switch.
In accordance with a concomitant feature of the invention, a fourth measurement device for measuring an actuator voltage is provided.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and circuit for driving at least one capacitive actuator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.